The Mesosphere Prof Nicholas Mitchell Centre for Space, Atmospheric & Oceanic Science Department of Electronic & Electrical Engineering The University of Bath Introductory Solar System Plasma Physics Summer School, Northumbria University, Sept. 2017 In this talk 1. The structure of the atmosphere 2. The mesospheric zoo – sprites, meteors, ghostly noctilucent clouds, giant waves and solar tides 3. The general circulation of the mesosphere – why radiative equilibrium isn’t enough 4. Driving the mesosphere by gravity waves 5. Science questions for the next decade 1. The structure of the atmosphere 2. The structure of the Atmosphere Spheres defined by temperature gradient Boundaries are the pauses Thermosphere: “hot sphere” Mesosphere: “middle sphere” Stratosphere: “stratified sphere” Troposphere: “turning sphere” Atmospheric density and pressure Reaching the mesopause – the edge of space The X-15 Rocket plane Spaceship one – a rocket plane to 100 km Upper atmosphere > ~ 100 km 2. The structure of the Atmosphere Common dynamics and chemistry defines lower, middle and upper atmosphere Middle atmosphere ~ 10 – 100 km Lower atmosphere ~ 0 – 10 km 2. The structure of Turbopause the Atmosphere Turbulent mixing stops at the turbopause at ~ 100 km Below is the homegeneous homosphere where turbulent mixing homogenises the major constituents – “well mixed”) Above is the hetrogeneous hetrosphere where molecular diffusion dominates and composition changes with height Various atmospheric properties as a function of height. Note how the composition changes significantly only at heights above ~ 100 km. Above ~ 180 km atomic oxygen is the major constituent. 2. The structure of the Atmosphere The ionosphere is the ionised component of the atmosphere, embedded in the neutral atmosphere 2. The mesospheric zoo – sprites, meteors, ghostly noctilucent clouds, giant waves and solar tides Sprites – lightning from thunderstorms into the mesosphere Sprites – lightning from thunderstorms into the mesosphere Sprites – lightning from thunderstorms into the mesosphere Meteors Meteors – sporadics and showers On a clear, moonless night, a naked-eye observer might see ~ 6 sporadic meteors per hour Meteors A Perseid meteor photographed from the ISS Height distribution of meteors Meteor Showers The Leonid meteor shower of 1833 was witnessed from Pennsylvania by Joseph Harvey Waggoner. Under his direction, artist Karl Jauslin produced this drawing. “Many persons thought that a shower of fire was falling and became exceedingly alarmed.” Naked-eye meteor count rates may have exceeded 100,000 per hour. Noctilucent clouds Noctilucent Clouds Typical NLC height ~ 82 km Polar Mesospheric Clouds – NASA AIM Long-term trends in NLC/PMC? No reliable reports of NLC observed before the late 880s There is good evidence of long-term trends in NLC/PMC occurrence rates, brightness and equator-ward extent These trends appear to a complex response to long-term trends in temperatures, solar activity and available water vapour A comparison of the seasonal PMC frequency of occurrence measured by SBUV and the fit to a linear regression in time and solar activity by latitude band (Shettle et al., GRL, 2009) Wave patterns in noctilucent clouds Photo: Alexander Lloyd-Ribeiro Gravity waves in tropospheric clouds (Gulf of Mexico) Wave patterns in a meteor trail Atmospheric solar tides over South Georgia, March 2016 Atmospheric solar tides in a model HOUR: GSWM APRIL TEMP MIGRATING 24-HR TIDE Perturbations in atmospheric temperature from the 24-hour tide Height ~ 111.5 km April conditions NCAR HAO Global Scale Wave Model Alternating “View fixed over point on Earth” and “View from Sun” frames of reference (http://www.hao.ucar.edu/public/research/tiso/gswm/gswm.html) Planetary Waves The atmosphere also supports planetary waves of many different types Rossby waves are a common type of planetary wave The figure shows Rossby wave perturbations in the upper tropospheric jet streams 3. The general circulation of the mesosphere – why radiative equilibrium isn’t enough The general circulation of the atmosphere Air moves in response to forces acting on air parcels. There are four main forces acting on the atmosphere that produce horizontal flows: 1. The pressure-gradient force – which pushes air from regions of high pressure towards regions of low pressure (NB – horizontal gradients) 2. The Coriolis force – which deflects motion to the right in the Northern Hemisphere and to the left in the Southern Hemisphere 3. Friction – which slows the flow speed when near the ground, particularly over rough terrain 4. Centripetal forces – only important in very tightly curved flows, such as tornados The general circulation of the troposphere is largely governed by pressure gradient forces, Coriolis forces and friction Consider the pressure-gradient force and the Coriolis force in more detail… 1. The pressure gradient force, Pn Variations in heating cause horizontal pressure gradients. The heating variations can be local (land/sea boundary) or global (equator/pole). In either case air attempts to flow away from regions of high pressure towards regions of low pressure in a circulatory cell. Example of pressure gradient forces – a sea breeze 1. 1. Before sunrise land and sea are at the same temperature. Pressure surfaces are horizontal and there are no pressure (horizontal) gradient forces 2. 2. At sunrise, the land warms more than the ocean. Air over the land expands and pressure surfaces move upwards to greater heights over the land. A horizontal pressure gradient arises that pushes air from high pressure to low pressure 3. 3. The accumulation of mass over the ocean results in higher pressure over the ocean surface and a circulation system is established 2. The Coriolis force The rotation of the Earth causes an apparent “force” which acts on all moving objects on the Earth. The force is proportional to velocity x sine(latitude). The force acts perpendicularly to the direction of motion - to the right in the Northern hemisphere and to the left in the Southern hemisphere. A conceptual explanation of the Coriolis force Imagine a projectile fired from the North pole such that it skims the Earth’ surface and lands on the equator. (a) Nonrotating Earth In this case the projectile flies a straight path over the surface of the Earth. (b) Rotating Earth In this case the rotation of the Earth means that the target has moved out of the way by the time the projectile reaches the equator. To an observer measuring the motion with respect to the Earth’s surface it appears as if a horizontal force has acted at 900 to the projectile’s motion and pushed it off to the right NB/ a similar argument reveals a deflection to the left for motion in the Southern hemisphere 2. The Coriolis force & geostrophic flow The effect of the Coriolis force is to deflect air flows so that they do not result in a direct high- pressure-to-low-pressure flow. Instead, the flow ends up being parallel to the pressure contours (isobars) at a given height. isobars The Coriolis force, C, is given by: C = 2vΩsinφ Where: v = velocity Ω = angular rotation rate of rotation of the Earth φ = latitude Geostrophic Flow An air parcel starts to accelerate under the action of the pressure gradient force (in this case Northwards). As the air parcel accelerates, the Coriolis force starts to deflect it to the right. A steady state is eventually reached in which the pressure gradient force is equalled and opposed by the Coriolis force. Flow where only the pressure gradient and Coriolis forces act is known as geostrophic flow North pole Geostrophic flow in the “cold” upper troposphere The equator-to-pole temperature gradient creates a corresponding Pn pressure gradient. In each hemisphere, the Coriolis v force, C, acts to balance the poleward pressure gradient force, P , and a stable East-West flow C n results “hot” Because the Coriolis force acts in an opposite sense in each C hemisphere, the geostrophic winds are westerly (i.e., eastwards) in v both hemispheres and in all seasons. Pn NB – this only applies to the “winds aloft” in the upper troposphere. At the surface it’s sometimes more complicated – e.g., seasonal “cold” monsoon circulations South pole North pole Northern- Geostrophic flow in the hemisphere “hot” strato/mesosphere summer The summer pole is hotter than the equator because it is in 24-hour C sunlight. The winter pole is cold. The pressure gradient force is thus v in the same direction in each hemisphere. Pn Because the Coriolis force acts in an opposite sense in each “warm” hemisphere, the geostrophic winds are easterly (i.e., westwards) in the C summer hemisphere and westerly (i.e., eastwards) in the winter v hemisphere. There is thus a seasonal reversal P of the mean winds (unlike the n troposphere) Southern- hemisphere “cold” winter South pole Radiative equilibrium predicted temperatures (solstice) SUMMER WINTER Notice: 1. Summer pole warmer than winter pole at all heights 2. Summer polar stratosphere heated by O3 absorption of solar UV Radiative equilibrium predicted geostrophic winds (solstice) SUMMER WINTER Notice: 1. Eastward winds in summer hemisphere, westward winds in winter hemisphere 2. Winds are zonal (i.e., E-W) only, there are no meridional (N-S) winds 3. Winds increase with height 4. There are no vertical winds Observed temperature structure (solstice) SUMMER WINTER Notice: 1. Meridional (N-S) temperature gradient is reversed in the mesosphere 2. Coldest mesopause temperatures in polar summer, warmest in polar winter Observed zonal winds (solstice) SUMMER WINTER Notice: 1. Eastward and westward winds still occur, but not as strong as predicted 2. But, winds reduce to zero near 90 km then actually reverse 3. There must be an additional force slowing then reversing the winds (“wave drag”) North pole Northern- Wave drag hemisphere “heated by sunlight” Atmospheric waves cause a summer drag force, F, on air parcels that opposes the motion. C Steady-state flow occurs when: P = C + F F n v Note how this occurs when there is a meridional flow away Pn from the summer pole towards the winter pole.